Many portable electronic devices, such as cameras, cellular telephones, Personal Digital Assistants (PDAs), MP3 players, computers, and other devices include an imaging device for capturing images. One example of an imaging device is a CMOS imaging device. A CMOS imaging device includes a focal plane array of pixels, each one of the pixels including a photosensor, for example, a photogate, photoconductor or a photodiode overlying a substrate for accumulating photo-generated charge in the underlying portion of the substrate. Each pixel has a readout circuit that includes at least an output field effect transistor and a charge storage region connected to the gate of the output transistor. The charge storage region may be constructed as a floating diffusion region. Each pixel may include at least one electronic device such as a transistor for transferring charge from the photosensor to the storage region and one device, also typically a transistor, for resetting the storage region to a predetermined charge level prior to charge transference, and a row select transistor for selectively connecting the pixel to a column line.
In a CMOS imaging device, the active elements of a pixel perform the necessary functions of: (1) photon to charge conversion; (2) accumulation of image charge; (3) resetting the storage region to a known state; (4) transfer of charge to the storage region accompanied by charge amplification; (5) selection of a pixel for readout; and (6) output and amplification of a signal representing a reset level and pixel charge. Photo charge may be amplified when it moves from the initial charge accumulation region to the storage region. The charge at the storage region is typically converted to a pixel output voltage by a source follower output transistor.
FIG. 1 illustrates a typical four-transistor pixel 50 utilized in a pixel array of an imaging device, such as a CMOS imaging device. The pixel 50 includes a photosensor 52 (e.g., photodiode, photogate, etc.), transfer transistor 54, and readout circuit 51. The readout circuit 51 includes a storage node configured as a floating diffusion region N, reset transistor 56, source follower transistor 58 and row select transistor 60. The photosensor 52 is connected to the floating diffusion region N by the transfer transistor 54 when the transfer transistor 54 is activated by transfer select line 53 carrying a transfer select signal TX. The reset transistor 56 is connected between the floating diffusion region N and an array pixel supply voltage Vaapix. A reset select signal RST supplied over a reset select line 57 is used to activate the reset transistor 56, which resets the floating diffusion region N to a known state as is known in the art.
The source follower transistor 58 has its gate connected to the floating diffusion region N and is connected between the array pixel supply voltage Vaapix and the row select transistor 60. The source follower transistor 58 converts the charge stored at the floating diffusion region N into an electrical output signal. The row select transistor 60 is controllable by a row select signal ROW supplied over a row select line 61 for selectively outputting the output signal OUT from the source follower transistor 58 to sample and hold circuit 46 via column line 45. For each pixel 50, two output signals are conventionally generated, one being a reset signal Vrst generated after the floating diffusion region N is reset, the other being an image or photo signal Vsig generated after charges are transferred from the photosensor 52 to the floating diffusion region N. Output signals Vrst,Vsig are selectively stored in the sample and hold circuit 46 based on reset and pixel sample and hold select signals SHR, SHS.
Conventional CMOS imager designs, such as that shown in FIG. 1 for pixel 50, provide only approximately a fifty percent fill factor, meaning only half of the pixel 50 layout area comprises a photosensor utilized in converting light to charge carriers. The remainder of the pixel 50 includes the transfer transistor 54 and the readout circuit 51. As the total pixel area continues to decrease due to desired scaling, it becomes increasingly important to create photosensors that utilize as much of the pixel surface area as possible to increase quantum efficiency.
Accordingly, there is a desire for a pixel array architecture which has an improved fill factor and increased quantum efficiency.